This invention relates to a vacuum-assisted braking booster, or a servobrake, comprising a pressure-measuring device.
A booster of a well-known type comprises a rigid jacket, made up of two shells forming, on the one hand, the cover and, on the other hand, the cylinder, which are assembled by crimping and on which the cover screws for the attachment to the dashboard cross-panel of the vehicle (rear part of the servobrake) and the cylinder screws for the fastening of the master cylinder (front part of the servobrake) are fixed respectively. Inside the jacket of the servobrake, a variable-volume front chamber is separated from a variable-volume rear chamber by a tight flexible diaphragm and by a rigid skirt driving a pneumatic piston which bears, through a push rod, on the primary piston of a tandem master cylinder of a hydraulic braking system. The front chamber, directed towards the tandem master cylinder, is hydraulically connected to a vacuum source whereas the rear chamber is hydraulically connected, using a valve control, to a propellant fluid source, typically atmospheric-pressure air.
At rest, that is when the driver does not depress the brake pedal, the front and rear chambers are interconnected, while the rear chamber is isolated from the atmospheric pressure. On braking, first the front chamber is isolated from the rear chamber and then air is admitted into the rear chamber.
In this type of servobrakes, it is most important to be able to measure the pressure in at least the rear chamber, by means of a sensor module, fastened on the front ace of the servobrake. Generally, the pressure transmission for the measurement is carried out using a helical tube, located inside the jacket.
With a view of, on the one hand, improving the rigidity and, on the other hand, reducing the weight of the servobrake, by means of a thinner jacket, through tie bars are generally used. Yet the assembling process is such that the use of a helical tube is incompatible with that of through-tie bars.
So as to cope with said disadvantages, it is the object of the present invention to provide a servobrake as per the preamble, which allows both the use of tie bars and a measurement of the pressure at least inside the rear chamber by means of a sensor situated on the front face and, moreover, an easy assembling of said servobrake.
To this end, the main subject of this invention is a servobrake, comprising a rigid jacket, made up of two shells, namely the cover and the cylinder, which are assembled by means of through-tie bars, and comprising variable-volume front and rear chambers, the front chamber being operatively permanently connected to a first pressure source whereas the rear chamber is connected to a second pressure source, said servobrake comprising a pressure sensor for the measurement of at least the pressure inside the rear chamber, characterized in that said pressure sensor is fastened to the front end of a through-tie bar, said tie bar having a hollowed-out body defining at least one chamber for the pressure transmission from the rear chamber towards the pressure sensor, said tie bar comprising ports connecting said rear pressure chamber with the transmission chamber.
Thus, in an advantageous manner, the through-tie bars having a hollowed-out body have substantially the same outer dimensions as the conventional solid through-tie bars, so that they can replace such conventional tie bars, when the servobrake is assembled and that, besides, they contribute to the simplifying of the construction of said servobrake in that they constitute a means for the pressure transmission from the rear chamber towards the pressure sensor, located at the front part of said servobrake, this being achieved with as few elements as possible.
According to a preferred embodiment of the invention, the hollowed-out through-tie bar further comprises a hollow inner needle, set in a leakproof manner in said body, in which case the through-tie bar includes, on the one hand, a chamber for the pressure transmission from the rear chamber towards the sensor, such chamber being formed by the hollow needle and, on the other hand, a chamber for the pressure transmission from the front chamber towards the sensor, such chamber being defined between the tie bar and the needle, and the tie bar comprises ports connecting said front pressure chamber with the transmission chamber, defined about the needle, as well as ports connecting the rear pressure chamber with the transmission chamber, formed in the needle.
Other features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments, by way of example and by no means as a limitation, when taken in conjunction with the accompanying drawings